1. Field of the Invention
The present invention relates to a heat-sensitive lithographic printing plate precursor, which requires no development-processing and can ensure a long press life and high stain resistance. More specifically, the present invention relates to a lithographic printing plate precursor which enables non-ablative recording of images by scanning exposure to radiation, such as infrared laser beams based on digital signals and, after the images are recorded therein, can be mounted in a printing machine (i.e. a printing press) without undergoing development-processing, and subjected to printing operations.
2. State of the Art
Various methods have been proposed concerning a lithographic printing plate precursor of the kind which enables image formation by heat and can be mounted in a printing machine without development-processing after the image formation. One method among them is a method of utilizing an ablation phenomenon, which comprises exposing a printing plate precursor containing a compound capable of converting light to heat by means of a high-output solid-state laser, e.g., a semiconductor laser or a YAG laser, to make the exposed area evolve heat by the compound capable of converting light to heat, thereby causing decomposition and evaporation, namely ablation, in the exposed area.
In negative-working compositions, a water-receptive layer is provided on a substrate having an oleophilic ink-receptive layer and the water-receptive layer is removed by ablation.
Reference is made to U.S. Pat. No. 6,397,749 and EP 1,110,719, both assigned to Fuji Photo Film, Ltd. which disclose heat sensitive lithographic printing plate precursors for on-press development, having a substrate supporting an ink-receptive or oleophilic layer and having thereover a crosslinked hydrophilic layer. In U.S. Pat. No. 6,397,749, a third water soluble overcoat layer is further provided to facilitate development on press.
Reference is also made to published EP Patent Applications 1,065,049 to 1,065,053 and to U.S. Pat. No. 5,985,515, all assigned to AGFA-Gevaert, N.V. These references also disclose heat-imageable lithographic printing plates having a support, a heat-sensitive oleophilic layer and a hydrophilic top layer containing a cross-linked hydrophilic polymer. In U.S. Pat. No. 5,985,515 and EP 1,065,052, the hydrophilic layer is heat-ablatable.
In EP 1,110,049 the hydrophilic layer contains heat-meltable, dispersed hydrophobic polymer particles which melt to form hydrophobic, oleophilic image areas. In EP 1,110,050 the thickness of the heat-sensitive oleophilic layer is regulated to permit the hydrophilic properties of the hydrophilic support to reduce toning in the non-imaged areas of the plate and thereby increase the run length of the imaged plate. In EP 1,110,051, the two-layer heat-sensitive material is covered by a third hydrophilic layer comprising an organic compound containing cationic groups.
A major disadvantage of ablation processes is contamination of the imaging device. Vacuum cleaning systems are generally required to avoid such contamination. Another disadvantage is that the decomposition of the plate layer(s) in the exposed areas results in the scattering of scum particles produced during decomposition, which scum particles attract printing ink to the non-image areas of the developed plate surface, resulting in background staining of the printed copies. This is alleviated to some extent by the application of a water-soluble overcoat layer.
It is also known in U.S. Pat. No. 6,107,001 to Presstek Inc., dated Aug. 22, 2000, to avoid the disadvantages of ablation or decomposition of the plate layers in the heat developed image areas by providing two layers, such as a hydrophilic top layer and an oleophilic bottom layer, the top layer becoming irreversably-debonded from the bottom layer in the heat-imaged areas.
None of the above cited references discloses interlayer chemical bonding between the hydrophilic and oleophilic layers.
The present invention relates to a radiation sensitive lithographic printing plate precursor preferably having only two polymeric layers on a support. The first (bottom) layer is composed of oleophilic polymer(s) and a photothermal converter which converts radiation to heat. The second polymeric layer (top) is composed of crosslinked hydrophilic polymer(s) which absorb aqueous fountain solution and repel ink. The oleophilic polymer(s) in the first layer contain functional groups, which chemically bond with the second layer to provide interlayer adhesive bonding between the two layers.
The plate is imagewise exposed to electromagnetic radiation, such as with an IR laser, resulting in non-ablative weakening of the adhesive bonding between the two layers so that it can be developed by fountain solution and/or ink on press. After development, the top layer in the exposed area is removed to reveal the underlying ink-receptive image area. The top layer in the unexposed area remains as non-image area.
The top hydrophilic layer has an opposite affinity for printing ink or ink-adhesive fluid from the heat-sensitive oleophilic, hydrophobic layer therebeneath. Preferably, both the top and bottom layers remain in place after the plate precursor is heat-imaged with no ablation of either layer. Any detachment of the top layer from the bottom layer in the heat-images areas is reversible to some extent by heating to elevated temperatures, as described in the examples.
The novel lithographic printing plate precursors of the present invention comprise:
(1) a substrate, which is dimensionally stable and can be aluminum or another metal or alloy. Preferentially electrochemically and/or mechanically grained and anodized aluminum is used in the present invention. Hydrophilization of the aluminum substrate is not required, but may be useful for enhancing adhesion of the base layer to the substrate.
Furthermore in connection with the present invention, the support can be a flexible support. As flexible support in connection with the present embodiment it is particularly preferred to use a polymeric film e.g. substrated polyethylene terephthalate film, polyethylene naphthalate film, cellulose acetate film, poly-styrene film, polycarbonate film, polyethylene film, polypropylene film, polyvinyl chloride film, polyether sulphone film. The plastic film support may be opaque or transparent. The polymeric film is preferably subbed with subbing layers as described in EP-A-619 524, EP-A-619 525 and EP-A-620 502.
Still further paper or glass of a thickness of not more than 1.2 mm can also be used.
(2) an olephilic base layer comprising an oleophilic polymer containing functional reactive groups, and a photothermal converter material, such as dye or pigment capable of absorbing electromagnetic radiation and converting it to heat, and
(3) a hydrophilic overlayer comprising a crosslinked hydrophilic polymer capable of absorbing aqueous lithographic fountain solution, in which said crosslinked hydrophilic polymer is interlayer chemically bonded with the functional reactive groups of said oleophilic polymer in the base layer to provide interlayer adhesive bonding between said layers, imagewise exposure of said printing plate precursor to laser radiation resulting in non-ablative weakening of said adhesive bonding between said layers in the exposed areas whereby said hydrophilic overlayer can be removed by fountain solution or ink on a lithographic printing press to reveal the oleophilic ink-receptive base layer.
Preferably the oleophilic polymer of the base layer composition and the crosslinkable hydrophilic polymer of the overlayer or top layer are both organic polymers.
Suitable oleophilic organic polymers for the base layer include acrylic polymers and copolymers, methacrylic polymers and copolymers, epoxy polymers, phenolic polymers, polyurethanes, polyureas and polyesters, most preferably such polymers contain at least one of xe2x80x94COOH and xe2x80x94OH functional reactive groups.
Suitable acrylic and methacrylic copolymers include copolymers with styrene, maleic acid, fumaric acid, itaconic acid and 2-carboxyethyl acrylate.
The oleophilic base layer is applied to the substrate at a coating weight, after drying, preferably within the range of about 0.8 to about 5.0 g/m2, more preferably about 1.0 to about 3.5 g/m2 and most preferably about 1.5 to about 3.0 g/m2. Lower coating weights of the bottom layer tend to reduce press life of the plates. The radiation-sensitive oleophilic layer comprises an oleophilic binder and a photothermal converter material capable of converting electromagnetic radiation into heat.
Suitable photothermal converters absorb UV, visible or infrared radiation and convert the absorbed radiation into heat. Preferred photothermal converter materials absorb radiation in the near infrared region in the range of about 800-1200 nm, more preferably in the range of about 810-900 nm. Useful compounds are for example dyes and in particular infrared dyes as disclosed in EP-A-908 307 and pigments, in particular infrared-absorbing pigments such as carbon black, metal carbides, borides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the A component e.g. WO2.9. It is also possible to use conductive polymer dispersions such as polypyrrole or polyaniline-based conductive polymer dispersions. The lithographic performance and in particular the print endurance obtained depends i.a. on the heat-sensitivity of the imaging element. In this respect it has been found that carbon black yields useful results. Most preferred photothermal converter materials are cyanine dyes.
Suitable hydrophilic organic polymers for the overlayer or top layer include hydroxy-functional polymers, carboxylic acid-functional polymers, sulfonic acid-functional polymers, amido-functional polymers and sulfonamido-functional polymers.
Preferred hydroxy functional hydrophilic polymers are selected from the group consisting of polyvinyl alcohol, a silanol-functional polyvinyl alcohol, a methacrylamido-functional polyvinyl alcohol, a sulfate-functional polyvinyl alcohol, an acrylic polymer, a methacrylic polymer, a polyester and mixtures thereof.
Preferred carboxy functional hydrophilic polymers are selected from the group consisting of polyacrylic acid, polymethacrylic acid, copolymers thereof and mixtures thereof.
Preferred amido functional hydrophilic polymers are selected from the group consisting of polyacrylamide, polymethacrylamide, copolymers thereof and mixtures thereof.
The crosslinked hydrophilic polymer of the overlayer or top layer preferably is formed from a crosslinkable hydrophilic polymer and a crosslinking agent which is further reactive with the functional reactive groups of the oleophilic polymer of the base layer. Suitable crosslinking agents will depend upon the specific hydrophilic polymer used in the top layer, and include a polyvalent metal salt, a polyvalent metal complex, an amino resin crosslinking agent, an amido resin crosslinking agent, an aldehyde crosslinking agent, an alkoxysilane crosslinking agent and a combination thereof. Preferably the polyvalent metal salt or said polyvalent metal complex contains a metal selected from the group consisting of magnesium, aluminum, calcium, titanium, ferrous, cobalt, copper, strontium, zinc, zirconium, stannous and stannic, most preferably a zirconium (IV) salt.
The hydrophilic top layer is applied over the oleophilic base layer at a coating weight, after drying, preferably within the range of about 0.5 to about 5.0 g/m2, more preferably about 0.8 to about 3.5 g/M2, most preferably about 1.0 to about 2.5 g/m2.
Most preferably, the printing plate precursor is one in which the oleophilic polymer comprises carboxylic acid groups, the hydrophilic polymer comprises hydroxy groups and the crosslinker is a zirconium (IV) salt or complex.
The oleophilic base layer can be applied to the substrate using application methods known in the art. For example, the components of the base layer, including an oleophilic polymer containing functional reactive groups and a photothermal converter, can be dissolved or dispersed in solvents, preferably organic solvents, and applied to an aluminum substrate, which has been preferably grained and anodized. Preferably the components of the base layer are dissolved in organic solvents. In a preferred embodiment the base layer coating solution is free of dispersed particles.
The hydrophilic top layer can be applied to the oleophilic base layer using application methods known in the art. For example, the components of the top layer, including a crosslinkable hydrophilic polymer, capable of interlayer chemical bonding with the oleophilic polymer in the base layer, can be dissolved or dispersed in solvents, preferably aqueous solvents. More preferably the components of the top layer are applied in water. In a preferred embodiment the top layer coating solution is free of dispersed particles.
After drying, the crosslinkable hydrophilic polymer in the top layer is crosslinked by the application of energy, which may be in the form of heat or electromagnetic radiation. The hydrophilic polymer in the top layer is also interlayer chemically bonded with the oleophilic polymer in the base layer to provide interlayer adhesive bonding. Interlayer chemical bonding is preferably covalent bonding, i.e. bonding involving the formation of interlayer covalent chemical bonds. Although generally weaker, interlayer electrostatic bonding, by attraction of unlike charges, and interlayer hydrogen bonding may also provide sufficient interlayer adhesive bonding.
Preferably, the top layer includes a crosslinker capable of crosslinking the hydrophilic polymer, as well as being capable of interlayer chemically bonding the hydrophilic polymer in the top layer with the oleophilic polymer in the base layer, by application of heat. In this case, sufficient heat is applied to accomplish the desired extent of crosslinking and interlayer chemical bonding, typically involving cure conditions with temperatures in the range of about 90 to about 160xc2x0 C., preferably about 120 to about 135xc2x0 C., and cure times in the range of about 0.5 to about 5 min, preferably about 1 to about 2 min.
The printing plate precursor can have additional layers, such as an underlying layer or an overlying layer. For example, an underlying layer may be useful for enhanced adhesion to the substrate. An overlying layer may be useful to prevent damage, such as scratching of the hydrophilic layer, and also to promote on-press developability. Preferably, the printing plate precursor is free of additional layers in order to enhance the efficiency and reduce the cost of manufacture.
The printing plate precursor is imagewise exposed with laser radiation, preferably in the near infrared region, for example, using a semiconductor laser or YAG laser, to selectively weaken the interlayer adhesive bonding between the oleophilic base layer and the hydrophilic top layer, non-ablatively, in the exposed areas. Some extent of non-ablative decomposition may also occur in the top layer during imagewise exposure. Following imagewise exposure, the imaged plate precursor can be mounted on a printing press, without an additional processing step, and contacted with fountain solution and ink, which selectively removes the hydrophilic top layer, thereby revealing the oleophilic base layer in the exposed areas, while retaining interlayer adhesive bonding in the unexposed areas, so that the resulting printing plate is capable of long press runs without background contamination of the printed copies.